Suspension system control responsive to ambient temperature

ABSTRACT

A suspension system control according to the steps of: determining an actuator demand force command; determining, responsive to vehicle speed, a first signal indicative of a first maximum demand force; determining, responsive to ambient temperature, a second signal indicative of a second maximum demand force; and constraining the determined actuator demand force command so that it is no greater than the lesser of the first and second maximum demand forces, wherein vehicle body isolation from suspension events is maintained during low temperature and low speed operation of the vehicle to thereby minimize suspension noise and ride harshness.

This invention relates to a variable force suspension system. Thesubject of this application is related to the subject of co-pendingUnited States patent applications, Ser. Nos. 08/410,794, 08/410,788,08/411,184 and 08/409,441, all filed concurrently with this application,all assigned to the assignee of this invention and all havingdisclosures that are incorporated herein by reference. This invention isrelated to the subject of pending U.S. patent application, Ser. No.08/358,925, filed on Dec. 19, 1994, and assigned to the assignee of thisinvention.

BACKGROUND OF THE INVENTION

Known variable force suspension systems include variable force shockabsorbers and/or struts that provide suspension damping forces at amagnitude controllable in response to commands provided by a suspensionsystem controller. Some systems provide control between two dampingstates and others provide continuously variable control of dampingforce.

In a known manner of control of a variable force suspension, the demandforce for each variable force damper is determined responsive to a setof gains, the wheel vertical velocity and the body heave, roll and pitchvelocities. An example system determines the demand force as follows:

    DF.sub.b =G.sub.h H'+G.sub.r R'+G.sub.p P'+G.sub.w v,

where DF_(b) is the demand force, G_(h) is the heave gain, G_(r) is theroll gain, G_(p) is the pitch gain, G_(w) is the wheel velocity gain, H'is the body heave velocity, R' is the body roll velocity, P' is the bodypitch velocity and v is the wheel vertical velocity. The portion of thedemand force computation, G_(h) H'+G_(r) R'+G_(p) P', represents thebody component determined responsive to the body heave, roll and pitchvelocities and the portion of the demand force computation G_(w) vrepresents the wheel component determined responsive to the differencebetween the computed body corner velocity and the body-wheel relativevelocity.

A control signal representing the determined demand force is output tocontrol the variable force damper responsive to the demand force.Example systems are described in U.S. Pat. Nos. 5,235,529, 5,096,219,5,071,157, 5,062,657, 5,062,658, all assigned to the assignee of thisinvention.

SUMMARY OF THE PRESENT INVENTION

Advantageously this invention provides a variable force suspensionsystem control that seeks to improve the performance and optimizecontrol of a variable force suspension system.

The suspension system control in accordance with the present inventionis characterized by the features specified in claim 1.

Advantageously, this invention provides a system for controlling avariable force suspension that introduces a limiting function, based onambient temperature and vehicle speed, for the variable force dampercommands.

Advantageously, this invention provides a suspension system control thatlimits the commandable damper force for the suspension system actuatorsin certain circumstances to eliminate unnecessary ride harshness,suspension noise, and to improve vehicle operator comfort withoutsacrificing suspension performance.

In a preferred implementation, this invention provides a suspensionsystem control according to the steps of: determining an actuator demandforce command; determining, responsive to vehicle speed, a first signalrepresentative of a first maximum demand force command; determining,responsive to ambient temperature, a second signal representative of asecond maximum demand force command; and constraining the determinedactuator demand force command so that it is no greater than the lesserof the first and second maximum demand force commands.

In another preferred implementation, this invention provides asuspension control system for use in a vehicle including a suspendedvehicle body, four un-suspended vehicle wheels, four variable forceactuators mounted between the vehicle body and wheels, one of thevariable force actuators at each corner of the vehicle, and a set ofsensors providing sensor signals indicative of motion of the vehiclebody, motion of the vehicle wheels, a vehicle speed and an ambienttemperature, the suspension control system comprising a microcomputercontrol unit including: means for receiving the sensor signals; means,responsive to the sensor signals, for determining an actuator demandforce for each actuator; means, responsive to the vehicle speed, fordetermining a first signal indicative of a first command maximum; means,responsive to the ambient temperature, for determining a second signalindicative of a second command maximum; and means for constraining theactuator demand force so that it is no greater than a lesser of thefirst and second command maximums.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 illustrates an apparatus according to this invention.

FIG. 2 illustrates an example control structure for a systemimplementing this invention.

FIG. 3 illustrates a block diagram for determining body velocities andcorner relative velocities for use with this invention.

FIGS. 4 and 5 are graphs of response of a filter implemented with thisinvention.

FIG. 6 illustrates an example portion of a control structure accordingto this invention.

FIG. 7 illustrates a control diagram for determining demand forcecommands according to this invention.

FIG. 8 illustrates a block diagram for determining body gains and demandforces according to this invention.

FIGS. 9, 10 and 11 and 12 illustrate graphs for selecting body gains inthe control according to FIG. 8.

FIG. 13 illustrates a control diagram for determining and scaling wheeldemand forces according to this invention.

FIG. 14 illustrates an example plot of calibration points to implement apassive wheel curve according to this invention.

FIG. 15 illustrates a flow diagram for implementing the passive wheelcurve according to this invention.

FIGS. 16 and 18 illustrate graphs for determining wheel control scalefactors according to the control of FIG. 13.

FIG. 17 illustrates a flow diagram for determining wheel demand forceaccording to this invention.

FIG. 19 illustrates a flow diagram for implementing phase-baseddetermination of total demand force according to this invention.

FIG. 20 illustrates a block diagram for limiting demand force accordingto this invention.

FIG. 21 illustrates a control diagram for determining a duty cycle forcontrol of a PWM controlled actuator according to this invention.

FIG. 22 illustrates a flow diagram according to this invention.

FIG. 23 illustrates a flow diagram for implementing environmentalcompensation according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an example apparatus for implementation of thisinvention is shown and, in general, comprises a vehicle body 10supported by four wheels 11 and by four suspensions including springs ofa known type (not shown). Each suspension includes a variable-force realtime controllable damper 12 connected to exert a vertical force betweenwheel 11 and body 10 at that suspension point. Although many suchsuspension arrangements are known and appropriate to this invention,actuator 12 of this embodiment comprises an electrically controllable,variable force damper in parallel with a weight bearing coil spring in aparallel shock absorber/spring or McPherson strut arrangement. Adescription of a variable force damper suitable for use as actuator 12is the continuously variable damper described in U.S. Pat. No.5,282,645, assigned to the assignee of this invention.

Each corner of the vehicle includes a linear position sensor 13 thatprovides an output signal indicative of the relative distance betweenthe vehicle wheel and the suspended vehicle body at that corner of thevehicle. Suitable position sensors 13 can be easily constructed by thoseskilled in the art. The outputs of the position sensors 13 may bedifferentiated to produce relative body-wheel vertical velocity signalsfor each corner of the vehicle and may be used, in the manner describedbelow, to determine the body modal velocities of body heave velocity,body roll velocity and body pitch velocity. The relative body-wheelvertical velocity signal is an example of what is referred to herein asa set of parameters indicative of motion of a body of the vehicle and ofmotion of wheels of the vehicle.

An example position sensor 13 includes a rotary resistive device mountedto the vehicle body and a link pivotably coupled between both thevehicle wheel and a pivot arm on the rotary resistive device such thatthe rotary resistive device provides an impedance output that varieswith the relative position between the wheel 11 and the corner of thebody 10. Each position sensor 13 may further include an internal circuitboard with a buffer circuit for buffering the output signal of therotary resistive device and providing the buffered signal to thecontroller 15. Suitable position sensors 13 can be easily constructed bythose skilled in the an. Any alternative type of position sensor,including transformer type sensors, may be used as position sensor 13.

The outputs of relative position sensors 13 are provided to a controller15 which processes the signals to determine the states of vehicle body10 and wheels 11 and generates an output actuator control signal foreach variable actuator 12. These signals are applied from controller 15through suitable output apparatus to control actuators 12 in real time.Input signals for the determination of the output actuator controlsignals may also be provided to microcomputer to provide anticipation ofvehicle pitch (lift/dive) 17 (described below with reference to FIG. 2)or by a vehicle speed sensor 18 and a steering wheel angular positionsensor 19 to provide anticipation of vehicle roll. Obtaining suchsignals is easily achieved through the use of known types of sensorsavailable to those skilled in the art.

Controller 15 is shown in more detail in FIG. 2. Signals from relativeposition sensors 13 are low-pass filtered through four analog low-passfilters 24 and differentiated through four analog differentiators 26 toprovide four relative velocity signals. An exemplary combination of sucha low pass filter and differentiator is shown in U.S. Pat. No.5,255,191, issued Oct. 19, 1993. The resulting relative velocity signalsrepresent the relative velocity between the front left wheel and thefront left corner of the body, rv₁, the rear left wheel and the rearleft corner of the body, rv₂, the front right wheel and the front rightcorner of the body, rv₃, and the rear right wheel and the rear rightcorner of the body, rv₄. Each of these relative velocity signals isinput to a digital microcomputer 22, which includes an input A/Dconverter 28 with multiplexed inputs; and each is digitally high-passfiltered within microcomputer 22 to remove any DC offset introduced bythe digitation of A/D converter 28. The relative velocity signals,processed as described below with reference to FIG. 3, are provided onbuses 76, 78 and 80, together with the body modal velocity signals onlines 70, 72 and 74 as a set of inputs to a control algorithm to helpdetermine the output actuator control signals for the vehicle suspensionsystem.

According to this invention, control of the variable force suspensionsystem actuator is not simply an additive control of body and wheelforce components, as is known in the prior art. While the prior artmethod of additive control of body and wheel force components was foundto improve suspension performance, this invention implements a method ofcontrol that offers further performance improvements includingminimization of the transfer of suspension harshness to the vehicle bodywithout sacrificing wheel control. This invention achieves theseadvantages by implementing a control method that treats the body andwheel effects on the suspension system separately and controls thevariable force actuators responsive to both the body and wheel effectsin a manner dependent upon the operating condition of the suspensionsystem as it affects the separately determined body and wheel controls.

Further in distinguishing from the prior art, the wheel controls are notdetermined in a linear manner such as by multiplication of a gain factorby the corner suspension relative velocity. Instead, the wheel controlis based on what is referred to herein as a passive curve. That is, thewheel control is a non-linear function of the relative velocityrepresented by DF_(n) =F(rv_(n)) (n=1, 2, 3, 4). Where F represents thenon-linear function that is implemented as described in more detailbelow via a look-up table.

Further, according to this invention, the wheel control is determinednot simply as a function of wheel speed, but is scaled as a function ofthe average vertical wheel velocity and also preferably by a factorresponsive to computed lateral acceleration of the vehicle. Thus, thewheel control component of this invention, determined according to theaverage vertical wheel velocity, has the capability of boosting thewheel control component when greater averages of vertical wheel velocityoccur indicating that the system is more entrenched in the wheel hopmode. Further, taking into account computed lateral acceleration of thevehicle offers the capability to boost wheel control during cornering.

According to this invention, the wheel and body force components are notjust simply added as done according to the prior art. Instead, accordingto the method of this invention, the wheel and body control componentsare combined in two different manners, based on two different possiblemodes of control. If the body and the wheel control components arein-phase, a first mode of control based on the wheel command as aminimum is used. That is, the demand force to the controllablesuspension system actuator is set equal to the body demand force if thebody demand force is equal to or greater than the wheel demand force,otherwise, the demand force for the actuator is set equal to the wheeldemand force. In this manner, the demand force to the actuator is alwaysequal to at least the wheel demand force.

If the wheel and body demand forces are out-of-phase, a second mode ofcontrol is used in which the wheel and body demand forces add in amanner similar to that known in the prior art. The result of this dualmode of control is both the reduction in body accelerations felt by thevehicle operator and the shortening of the time required for thesuspension system to move from wheel hop mode to body mode for improvedvehicle performance.

Referring to FIG. 2, a general block diagram of the control of thisinvention illustrates the processing of the input signals and the outputof the control signal for control of the variable force actuators. It isassumed that the actuators are controlled by a PWM control. However,actuators of another type not based on PWM control can be substituted asan alternative and it will be recognized that variable force controlsother than those with PWM control are equivalents to the PWM controlexample set forth herein.

The control is performed by a microprocessor suitable for providing PWMcontrol output. Such microprocessors are known and readily available tothose skilled in the art. The output of the control shown in FIG. 2, thePWM duty cycle commands on lines 112, 114, 116 and 118, may be in theform of signals representing duty cycles that standard microprocessorsreadily convert to the proper duty cycle PWM output, for example,vis-a-vis a standard PWM output interface 111.

Reference 50 represents four input signals of the relative velocities ofthe four corner suspensions of the vehicle. The relative velocities aredetermined from the position sensors 13 by low pass filtering theoutputs of sensors 13 through four analog low pass filters 24. Thefiltered outputs are then differentiated through four analogdifferentiators 26 to provide the four relative velocity signalsrepresenting the relative velocity between the front left wheel andfront left corner of the vehicle body rv₁, the rear left wheel and rearleft corner of the body rv₂, the front right wheel and front rightcorner of the body rv₃ and the rear right wheel and rear right corner ofthe body rv₄. Each of these relative velocity signals is input into themicroprocessor 22 through an A/D converter and result as the signals onlines 50 shown.

In an alternative example implementation, relative position sensors 13are replaced with relative velocity sensors of a type known to thosekilled in the art capable of outputting a signal indicative of therelative velocity between each wheel and corner of the vehicle body. Inthis alternative, there is no need for the differentiators describedabove used to convert the signals from sensors 13 to relative velocitysignals.

Various discrete signals are provided to the microprocessor input/outputport, illustrated schematically as reference 67. Line 52 carries asignal representing the vehicle speed and is preferably buffered in aknown manner to remove unwanted noise. Line 54 represents an input ofcomputed lateral acceleration of the vehicle that is computed in a knownmanner (block 55) based on the steer angle of the front wheels (on line53), which may be determined from sensing the position of steering wheel19 (FIG. 1), and based on vehicle speed. Line 56 represents an inputfrom a standard diagnostic routine (block 59) that performs knowndiagnostic functions such as checking for open circuits and shortcircuits from the sensors 13 or actuators 12 or any of the other inputlines (represented in general as bus 61). In response to a diagnosticfailure command on line 56, control block 110 forces a default output onlines 112, 114, 116 and 118 to control the actuators in a default mode.

Input line 58 is a digital signal representing the battery voltage thatis input through the microprocessors A/D converter 28 and is used atblock 92 to scale the duty cycle commands responsive to the batteryvoltage. The signal on line 60 is a signal that indicates the vehicle isin a dive (from end dip) or lift (from end rise) tendency situation suchas occurs during hard braking or hard acceleration of the vehicle. Thissignal may be provided by a powertrain controller that determines avehicle dive tending situation if a decrease in vehicle speed over apredetermined time period is greater than a predetermined limit anddetermines a lift tending situation if an increase in throttle angleover a predetermined time period is greater than a predeterminedthreshold. In general, the signal on line 60 is active when there iseither a detected lift or dive, and is otherwise inactive.

The signal on line 62 is representative of the ignition voltageavailable when the vehicle is keyed on and is de-bounced in a knownmanner. The signal on line 64 indicates whether or not the vehicle ismoving (block 69), for example, line 64 goes high when the vehicle speedis greater than 3 miles per hour. Line 66 is an override line that canbe used for in-plant testing of the system. For example, a signal online 66 can cause the dampers to cycle and the service operator can testthe dampers to determine if they are cycling properly.

In general, the corner relative velocity signals on lines 50 are inputto the signal conditioning block 68, which performs the functionsillustrated in more detail in FIG. 3 to provide signals on lines 70, 72and 74 of the vehicle body heave, roll and pitch velocities, on bus 76(four signals), which are high pass filtered relative velocity signalsfor the four comers of the vehicle, on the bus 78 (four signals), whichare lead-lag filtered relative velocity signals, and on the bus 80 (fourlines), which represent the average vertical velocities of the fourwheels of the vehicle.

More particularly, referring now also to FIG. 3, a determination of thesignals on lines 70-80 can be better understood. At block 122 therelative velocity input signals from the microprocessor's A/D converterare offset by a predetermined amount to compensate for the offsetintroduced during the A/D conversion. Further, block 122 scales therelative velocity signals to a scale, easily predetermined by a systemdesigner, so that the scaled value represents wheel motion. The scalingis achieved by simply multiplying the offset results by a predeterminedscaling factor. The results of block 122 are the offset, scaled relativevelocity signals on lines 124. The signals on lines 124 are provided toblock 126, which performs a digital high pass filtering of the signalson lines 124 to further remove any DC offsets introduced by thedigitation of the A/D converter. The resultant high pass filteredrelative velocity signals for the four corner suspension are provided onlines 128, 130, 132 and 134 to the three block 135, 136 and 138, whichperform the average vertical wheel velocity determinations, the bodymodal velocity determinations and the relative velocity lead-lagfiltering respectively.

More particularly, block 135 determines the average vertical wheelvelocity by first high pass or band pass filtering separately each ofthe signals on lines 128-134 to isolate the wheel content of eachrelative velocity signal from the body content. That is, the high passor band pass filter eliminates the 1 Hz. component while passing the 10Hz. component of the signal. Next, the output of the high pass filter isrectified or its absolute value is taken in order to obtain themagnitude information of the output signal. Then, to integrate oraverage the magnitude information, a low pass filter is applied to eachof the rectified signals so that, as a result, each of the lines 80carries a signal indicative of the average wheel velocity of one of thefour wheels.

An example high pass filter for isolating the wheel component of eachsignal on lines 128-134 can be implemented using the equation:

    V.sub.av (k)=rv.sub.x (k)-rv.sub.x (k-1)+a*V.sub.av (k-1),

where V_(av) is the average vertical wheel velocity, k represents thecurrent sampling period, rv_(x) is the suspension relative velocity forcorner x of the vehicle, and a is a predetermined constant, for example,0.90.

An example low pass filter may be implemented according to the equation:

    V.sub.av (k)=b.sub.*rvx (k)+(1-b)V.sub.av (k-1),

where b is a constant with example values equal 0.01, 0.002 and 0.001.

Referring now to block 136, the determination of body heave, roll andpitch velocity is carried out in the manner described in theabove-referenced pending patent application, Ser. No. 08/358,925. Moreparticularly, block 136 receives the relative velocity signals on lines128-134 and first performs a set of geometric transforms to obtain therelative states of relative heave velocity, relative roll velocity andrelative pitch velocity between the vehicle body and wheels.

The geometric transforms used to obtain the relative heave velocity(rv_(H)), relative roll velocity (rv_(R)), and relative pitch velocity(rv_(P)) are implemented respectively as follows:

    rv.sub.H (rv.sub.1 +rv.sub.2 +rv.sub.3 +rv.sub.4)/4

    rv.sub.R =(-rv.sub.1 +rv.sub.3)/tw,

    rv.sub.P =(-rv.sub.1 +rv.sub.2 -rv.sub.3 +rv.sub.4)/(2*wb),

where tw is the average track width or wheel span of the vehicle and wbis the wheel base of the vehicle. In the determination of rv_(R), onlyrv₁ and rv₃ are used in vehicles in which flexing or noise of the rearsuspension affects the quality of the relative roll velocity determinedusing rv₂ and rv₄. If rear suspension flexing or noise does not affectthe relative roll velocity determination, then the determination ofrv_(R) can be set forth according to:

    rv.sub.R =(-rv.sub.1 -rv.sub.2 +rv.sub.3 +rv.sub.4)/(2*tw).

Once each of the transforms is completed, a digital low pass filterfilters each of the relative heave, roll and pitch velocities to deriveaccurate estimates of the body heave, roll and pitch velocity.

The low-pass filters provide a significant amplitude roll-off above the1 Hz signals typical of resonant body modal vibrations so as to suppressthe 10 Hz signals typical of resonant vertical wheel vibrations and thusyield a signal with information about the amplitude of vehicle bodymotion in the heave, roll and pitch modes. However, for such signals tobe usefully applied in a real time control system, their phase iscritical. Low pass filters tend to produce a phase lag in the signal andthis phase lag increases across the frequency spectrum through a rangethat increases with the number of filter poles. In order to produceeffective control, in real time, of rapidly moving suspensioncomponents, the control signal must be applied in correct phaserelationship thereto. In order to emulate the integrated output of anabsolute accelerometer, a phase lead of approximately 90 degrees isrequired, while a low pass filter generally produces a phase lag.However, it has been found that a two pole low pass filter applied tothe relative body modal velocity signal will produce a signal with a 90degree phase lag, which can be inverted to provide the required 90degree phase lead.

A suitable second order low pass filter for use in the estimation isdescribed as:

    H.sub.Q (s)=K.sub.Q [ω.sub.o.sup.2 /(s.sup.2 +(ω.sub.o /Q)s+ω.sub.o.sup.2)],

where K_(o) is the filter gain, ω_(o) describes the filter polelocations in radians and Q is the filter quality factor.

Each low pass filter may be adjusted independently to tune the resultantmodal velocity estimates to match in magnitude and, with an additionalphase inversion, in phase, signals achieved by the prior art utilizingaccelerometers or to match the performance of another device such as anexternal velocity sensor or gyroscope from which the same measurementsmay be obtained. The values of the filter gains, pole locations andquality factor tend to change from vehicle type to vehicle type due todifferences in the natural body and wheel frequencies of differentvehicle models. The heave, roll and pitch velocities obtained areestimations since they are derived from relative measurements and notfrom absolute measurements. In one example, the continuous timeparameters for the low pass filter are shown in the following table:

    ______________________________________                                        MODE       K.sub.q        ω.sub.o                                                                       Q                                             ______________________________________                                        HEAVE      -13.889        3     0.35                                          ROLL       -18            5     0.50                                          PITCH      -7.8125        4     0.40                                          ______________________________________                                    

Typical magnitude and phase frequency responses for the two pole,low-pass filters are shown in FIGS. 4 and 5, respectively. FIG. 4 showsthe roll-off in amplitude above 1 Hz, which provides an output signalhaving mainly vehicle body movement information. FIG. 5 shows the phaseresponse of a two pole, low pass filter having an additional phaseinversion. Without the additional phase inversion, the phase curve wouldbe shifted downward by 180 degrees--starting at zero degrees at the leftside of the graph and decreasing with increasing frequency--to provide a90 degrees phase lag at 1 (10°) Hz. With the additional phase inversion,however, the curve is shifted upward to provide a 90 degree phase leadat 1 Hz. Thus, the filter output has the desired phase relationship atthe frequency of body resonance. In the table produced above, theadditional phase inversion is achieved through the minus sign of theconstant K_(q). The results of block 136 are the outputs of body heave,roll and pitch velocities on lines 70, 72 and 74. The lead-lag filter atblock 138 receives the relative velocity signals on lines 128-134,filters the signals and adds an approximate lead to the signal tocompensate for phase lag that may be introduced within the system,including that of the differentiator circuit, at the expense of acertain degree of magnitude distortion. More particularly, block 138functions as follows: A single pole infinite impulse response high-passfilter is used to provide a desired amount of phase lead at wheel hopfrequencies. Specifically, the filter may be implemented using thefollowing transfer function:

    H(z)=(LLA-LLBz.sup.-)/(1-LLCz.sup.-4),

where H(z) is a discrete, or "z" domain transfer function relating thez-transform of the filter output to that of the filter input. LLA, LLBand LLC are calibration constants stored in memory.

In an example, assume that at a wheel-hop frequency of 14 Hz,approximately 20 degrees of phase lag is introduced into the relativevelocity signal due to the system filtering, physical lag in the damperand other system lags. To compensate for the phase lag, approximately 20degrees of phase lead at wheel-hop frequency of 14 Hz is introduced bysetting the coefficients LLA, LLB and LLC to calibrated values of1.9521, 1.8564 and -0.9043.

The lead-lag filter transfer function may be implemented as follows:

    Y(k)=LLA*X(k)-LLB*X(k-1)+LLC*Y(k-1),

where LLA, LLB and LLC are in Q15 format, Y(k) is the LF, RF, LR or RRlead-lag filtered relative velocity output, Y(k-1) is the Y(k) from theprevious filter loop, X(k) is the LF, RF, LR or RR high-pass filteredrelative velocity (the filter input), and X(k-1) is the X(k) from theprevious loop of the filter.

The outputs of block 138 are the lead-lag filtered relative velocitysignals for each corner of the vehicle on lines 78.

Referring again to FIG. 2, block 102 receives the lift/dive discretesignal on line 60 and performs a de-bounce function of a known type,providing the de-bounced lift/dive signal on line 104 to block 110.Block 110 (1) further buffers the lift/dive signal, (2) determineswhether or not a floor profile output override is active and, if so,responsive to the lift/dive signal, determines what PWM duty cycle touse as the floor (minimum duty cycle) and (3) applies the time-varyinglift/dive PWM duty cycle floor as the minimum PWM duty cycle.

The buffer limits to 10 seconds (as an example) the amount of time thatthe control system will follow a lift/dive signal. Thus, if a shortcircuit causes an erroneous lift/dive signal, the system is onlyaffected for 10 seconds. The limitation on the effect of the lift/divesignal is achieved by setting a timer when the de-bounced lift/divesignal becomes active (a lift or dive is occurring) and forcing thelift/dive signal to be inactive if, after 10 seconds, the lift/divesignal remained continually active.

Whenever the buffered lift/dive signal is active, an associated minimumPWM duty cycle, referred to as the lift/dive PWM duty cycle floor, iscomputed as a function of time and the last inactive occurrence of thebuffered lift/dive signal. When the buffered lift/dive signal is active,the lift/dive PWM duty cycle floor is set equal to a calibratedlift/dive floor stored in memory. The calibrated lift/dive floor iseasily determined by one skilled in the art as the minimum PWM dutycycle desired during acceleration of the vehicle and will vary fromvehicle to vehicle based on desired operator "feel" or ride comfort andperformance.

When the buffered lift/dive signal is not active, the lift/dive PWM dutycycle floor is computed as a percentage of the lift/dive floorcalibration based on the ratio relating the time since the last activesignal to the lift/dive hold time calibration. The lift/dive hold timecalibration is a predetermined constant setting the decay time of thelift/dive PWM duty cycle floor and can be easily determined by a systemdesigner to provide a desired operator feel or vehicle performance.

FIG. 6 illustrates how the lift/dive PWM duty cycle floor is determined.Block 440 receives the buffered lift/dive signal on line 104 and theduty cycle floor and hold time calibrations from memory on lines 442 and444. The buffered lift/dive signal on line 104 appears as trace 446 andthe output lift/dive PWM duty cycle floor signal on line 450 obtains theshape of trace 448. When the buffered lift/dive signal is active, thesignal on line 450 is equal to the lift/dive PWM duty cycle floorcalibration and, after the buffered signal becomes inactive, the signalon line 450 decays over a time period equal to the hold time.

The signal on line 450 is applied at block 110 (FIG. 2) to limit theminimum PWM duty cycle of the actuator commands. Any actuator commandwith a duty cycle lower than the signal on line 450 is automatically setequal to the signal on line 450. Otherwise, the actuator commands arenot affected by the signal on line 450. When there is no lift/diveactivity, the signal on line 450 remains low and does not affect theactuator PWM commands.

Block 106 (FIG. 2), the mode control block, receives the ignition onsignal on line 62 (which is preferably de-bounced) and, if the ignitionon signal is active (indicating that the vehicle is on), outputs asignal on line 108 that enables the outputs of block 110. Without theenable signal on line 108, any commands determined will not be output onlines 112-120 and the controller is allowed to enter a standard "asleep"state of the type used in automotive controllers when the vehicleignition is off. The signal on line 108 does not force any outputcommand levels, but simply enables commands to be output from block 110.

Block 82 represents the control algorithm for implementing the variableforce suspension control according to this invention and outputs onlines 84, 86, 88 and 90, filtered PWM duty cycle commands representingthe force commands for each of the variable force actuators.

More particularly, referring now also to FIG. 7, block 82 is shown inmore detail and includes body control block 142, which receives the bodymodal velocities on lines 70-74, the vehicle speed signal on line 52 andthe computed lateral acceleration on line 54 and outputs a set of bodydemand forces on bus 164 (four lines) and gain table pointer on line166. Block 142 will be explained in more detail now with reference toFIG. 8.

The body control, block 142, includes body gain table switching block224, steering gain table switching block 244, gain sorting block 258 andblock 260, which computes the body demand force for each corner of thevehicle. More particularly, block 224 receives the body heave, roll andpitch velocities on lines 70, 72 and 74 and the buffered vehicle speedsignal on line 52 and receives from memory the calibrations on lines146-154.

FIGS. 9, 10 and 11 illustrate example gain table implementations atblock 224. In FIG. 9, vehicle speed is plotted against body heavevelocity, in FIG. 10, vehicle speed is plotted against body rollvelocity and, in FIG. 11, vehicle speed is plotted against body pitchvelocity.

The calibration inputs on line 146 (FIG. 8) include the ISO/body speedcalibration, which is a vehicle speed below which the isolation gaintable is selected when neither heave, roll nor pitch velocity are abovethe outer switch point (OSP) 1. Line 148 represents the OSP points 1-4for each of the heave, roll and pitch tables. The OSP points are thepoints above which the heave, roll or pitch velocities must rise beforeswitching to the next highest body table. Thus, if the vehicle speed isbelow the ISO/body shift speed for the table shown, heave velocity mustbe above OSP 1 for the body 1 table flag to be set, above OSP 2 for thebody 2 table flag to be set, above OSP 3 for the body 3 table flag to beset and above OSP 4 for the body 4 table flag to be set.

The calibrations also include the hold times represented by line 150that operate as follows. When each body table flag is set, the routineis prevented from resetting that flag until after that flag has been setfor its corresponding hold time. This prevents sporadic movement intoand out of one of the body tables. Lines 152 and 154 represent the innerswitch point (ISP) factors for the body tables. The ISP factors are thefactors below which the velocity for that table must fall before theflags for that body table are reset. The ISP references are slightlybelow the corresponding OSP references to provide hysteresis in theswitching between the tables.

The outputs of the body gain table switching block 224 are the flags forbody 1, body 2, body 3 and body 4 tables on lines 236, 238, 240 and 242,respectively. The isolation table is selected when all of the body 1-4table flags are reset. The flags on lines 236-242 are provided to thesorting block 258 along with the flags from block 244 on lines 246 and248.

Block 244 selects flags that point to gain tables to take into accountvehicle steering maneuvers. Input to block 244 are the buffered speedsignal on line 52 and the vehicle lateral acceleration signal on line54. While, as indicated above, lateral acceleration on line 54 may becomputed responsive to vehicle speed and steer angle, it can also beprovided from an accelerometer that measures actual vehicle lateralacceleration. Block 244 performs the single table function shown in FIG.12 in which vehicle speed is plotted against lateral acceleration.Calibration inputs to block 244 are the steering OSP points 3, 2 and 1(line 156) above which the lateral acceleration must rise to set thesteering gain flags in the speed regions shown. The vehicle speedregions are divided between 0 and speed 1 (SPD 1), speed 1 and speed 2(SPD 2), and above speed 2. The calibrations for speed 1 and speed 2 areprovided by line 158 from microprocessor memory. The ISP factors or thefactors below which the lateral acceleration must fall to reset anyflags are represented by lines 160 and the hold time that is the minimumtime for which a flag once set must remain set, is represented by line162.

If the vehicle speed is below speed 1 and lateral acceleration risesabove OSP 1, the steering table 1 flag is set. If the vehicle speed isbetween speed 1 and speed 2 and the lateral acceleration rises above OSP2, then the steering table 1 flag is set. If the vehicle speed is abovespeed 2 and the lateral acceleration rises above OSP 3, then thesteering table 2 flag is set. The steering table 1 and 2 flags areprovided on lines 246 and 248.

At block 258, a sorting operation provides a gain table pointer outputthat selects the gain table to be used to compute the body demand forcefor each actuator at block 260. The sorting is provided according to asimple hierarchy as follows. In the absence of any flags on lines236-242 and lines 246 and 248, the sorting defaults the pointer to theisolation table. The remaining possible pointing positions are given thefollowing priority: the steering table 2 flag is given highest priority,the steering table 1 flag is given next highest priority, followed bythe body table 4, body table 3, body table 2, body table 1 and thedefault and lowest priority table is the isolation table (no flags set).The flag with the highest priority controls the gain table pointer sothat the gain table pointer points to the table corresponding to theflag with the highest priority.

Responsive to the gain table pointer signal on line 166 and the heave,roll and pitch velocities on lines 70, 72 and 74, respectively, block260 computes body demand forces for each corner of the vehicle asfollows. The gain table pointer 166 controls the set of gains that areselected from the computer memory represented by line 144. The selectedgains and the computed body heave, roll and pitch velocities on lines70-74 are then used to determine the body demand force as follows:

    DF.sub.b =G.sub.h H'+G.sub.r R'+G.sub.p P'

where DF_(b) is the body demand force, G_(h) is the heave gain, G_(r) isthe roll gain, G_(p) is the pitch gain, H' is the body heave velocity,R' is the body roll velocity and P' is the body pitch velocity. The bodydemand force DF_(b) is computed for each corner of the vehicle so thatfour body demand forces DF_(b1), DF_(b2), DF_(b3) and DF_(b4) arecomputed. The gains G_(h), G_(r) and G_(p) for the front typically willvary from the gains G_(h), G_(r) and G_(p) for the rear. The results ofblock 260 are the four body demand force signals provided in the fourline bus 164.

Referring now again to FIG. 7, the wheel control block 168 responds tothe buffered vehicle speed signal on line 52, the lateral accelerationon line 54, the four average wheel velocity signals on lines 80 and thefour lead-lag filtered relative velocity signals on lines 78 to providethe wheel demand force signals, one for each corner of the vehicle, onbus 170.

More particularly, referring now also to FIG. 13, a more detailedillustration of the wheel control block 168 is shown. The wheel controlblock 168 includes block 278, which passively performs a non-linearfunction, once each on each of the lead-lag filtered relative velocitysignals on lines 78. Line 182 represents the stored calibration pointsfor the passive curve in the microcomputer memory. For example, thecurve might include seven positive relative velocity calibration pointsand seven negative relative velocity calibration points that plot anon-linear curve. Different curves may be implemented for the front andrear suspensions.

Referring to FIGS. 14 and 15, an example plot of six positive and sixnegative calibration points on a relative velocity versus demand forcecurve and an example flow diagram for implementing the passive wheelcurve are is illustrated. The passive curve function is implemented asfollows for each lead-lag filtered relative velocity signal. At block380, the stored calibration points for the non-linear curves areretrieved from computer memory. At block 382, the lead-lag filteredrelative velocity signal is compared to the calibration points and thetwo points immediately above and below the lead-lag filtered relativevelocity are selected and designated as v₁, DF₁ and v₂, DF₂. Then, thewheel demand force is determined at block 384 by the interpolationequation:

    DF.sub.w =DF.sub.1 +(DF.sub.2 -DF.sub.1)(v.sub.LLF -v.sub.1 /(v.sub.2 -v.sub.1)

where DF_(w) is the wheel demand force, v_(LLF), is the lead-lagfiltered relative velocity, v₁ is the lower bounding relative velocitycalibration point, v₂ is the upper bounding relative velocitycalibration point, DF₁ is the demand force calibration corresponding tov₁ and DF₂ is the demand force calibration corresponding to v₂. Note: v₁<v_(LLF) <v₂ and DF₁ <DF_(w) <DF₂. Block 386 stores the demand forceDF_(w) and the routine is ended at block 388. In the above manner, thelinear interpolation between the calibration is performed for eachlead-lag filtered relative velocity signal and the results on lines 280,282, 284 and 286 are the raw wheel demand force signals for each wheel.

Block 264 determines scale factors for each of the raw wheel demandforce signals responsive to the average vertical wheel velocity signalson lines 80. For each of the lines 80, the scale factor is determinedaccording to the graph shown in FIG. 16 responsive to the averagevertical wheel velocity input, v_(av). Line 172 represents the two scalefactors SF_(av1) and SF_(av2) calibrated in computer memory and line 174represents the average speed values AV1 and AV2 that determine the scalefactor that is used. Referring now also to FIG. 17, the storedcalibration values SF_(av1), SF_(av2), AV1 and AV2 are retrieved frommemory at block 400. If the average vertical wheel velocity is below AV1at block 402, then block 404 sets the average vertical velocity scalefactor SF_(av), equal to SF_(av1). If the average vertical wheelvelocity for that corner of the vehicle is above AV2 at block 406, thenblock 408 sets the average vertical velocity scale factor SF_(av), equalto SF_(av2). If the average vertical wheel velocity is not greater thanAV2 and not less than AV1, then block 410 determines SF_(av) as a linearinterpolation between AV1 and AV2 according to:

    SF.sub.av =SF.sub.av1 +(SF.sub.av2 -SF.sub.av1 (v.sub.av -AV1)/(AV2-AV1)

A scale factor is determined for each corner of the vehicle and thescale factors are output on lines 266, 268, 270 and 272 to block 288(FIG. 13).

Block 274 determines a single scale factor referred to as the lateralacceleration scale factor. The lateral acceleration scale factor isdetermined responsive to the speed signal on line 52 and the lateralacceleration signal on line 54 and is output on line 276. Line 176represents the two scale factor calibrations programmed intomicroprocessor memory, line 178 represents the two lateral accelerationthreshold points programmed into microprocessor memory and line 180represents the speed dead band programmed into microprocessor memory.

Referring now also to FIG. 18 and again to FIG. 17, the calibrationsLA1, LA2, v_(db), SF_(LA1) and SF_(LA2) are retrieved from memory atblock 412. If at block 414, the vehicle speed V_(veh) is less than thedead band speed, v_(db), for example, 20 m.p.h., then the scale factorSF_(LA) is set equal to SF_(LA1) at block 416, where SF_(LA1) is one ofthe two calibrated scale factors provided on line 176 together withSF_(LA2).

If the vehicle speed is above the dead band and lateral acceleration,LA, is below the calibrated value LA1 at block 418, then block 416 setsthe lateral acceleration scale factor equal to SF_(LA1). If the lateralacceleration is greater than the calibrated point LA2 at block 420, thenblock 422 sets the lateral acceleration scale factor equal to the valueSF_(LA2). If the lateral acceleration is not less than LA1 and notgreater than LA2, then block 424 determines the lateral accelerationscale factor, SF_(LA), as a linear interpolation between LA1 and LA2according to the equation:

    SF.sub.LA =SF.sub.LA1 +(SF.sub.LA2 -SF.sub.LA1)(LA-LA1)/(LA2-LA1))

The resultant lateral acceleration scale factor SF_(LA) is output online 276 to block 288 (FIG. 13).

Block 288 determines the scale factor to be applied to each wheelresponsive to the scale factors on lines 266, 268, 270, 272 and 276 bychoosing, for each wheel, the greater of the wheel velocity scale factorfor that wheel and the lateral acceleration scale factor and providingthat selection on the appropriate output line 290, 292, 294 and 296.This is performed at block 426 (FIG. 17) which compares SF_(LA) toSF_(AV). If SF_(LA) is greater than SF_(AV), then block 428 sets thescale factor SF equal to SF_(LA). Otherwise, block 430 sets the scalefactor equal to SF_(AV). At block 298 (FIG. 13, corresponding to block432, FIG. 17) the scaled wheel demand forces are determined bymultiplying each raw wheel demand force on lines 280-286 by thecorresponding scale factor on lines 290-296. Block 434 stores the scaledwheel demand forces in memory and the routine in FIG. 17 ends at block436.

The routine illustrated in FIG. 17 is performed once for each corner ofthe vehicle to determine the four scaled wheel demand forces. Theresultant scaled wheel demand forces are output on bus 170 (FIG. 13).

Referring now again to FIG. 7, block 184 determines the total demandforce for each corner of the vehicle responsive to the four body demandforces on bus 164, the four wheel demand forces on bus 170 and the fourlead-lag filtered relative velocity signals on lines 78. While theexample below is determined responsive to the lead-lag filtered relativevelocity signal, if a lead-lag filter is not implemented, a non-lead-lagfiltered relative velocity signal may be implemented instead.

More particularly, block 184 determines the total demand force,DF_(tot), for each actuator by combining the wheel and body demandforces, DF_(w) and DF_(b), respectively, based upon the phase of thedemand forces. If the body and wheel demand forces are in-phase, thedemand force is determined according to a wheel force as a minimumapproach. That is, the demand force is set equal to the body demandforce unless the body demand force is less than the wheel demand force,in which case, the total demand force is set equal to the wheel demandforce. If the wheel and body demand forces are out of phase, the wheeland body demand forces are summed to generate the total demand force.

Damper systems that do not provide energy from another source, such as acompressor storing high pressure hydraulic fluid, are referred to aspassive, meaning they can only generate force that is the same sign(direction) as the relative velocity of the damper. Active dampers thathave external supplies of energy can supply force independent of therelative velocity of the damper. On a force-versus-relative velocityplot, the passive quadrants are the first and third quadrants, that is,the quadrant in which relative velocity and force are both positive andthe quadrant in which relative velocity and force are both negative. Inthe second and fourth quadrants, a passive damper cannot provide acommanded force.

The wheel demand force, according to this invention, is a function ofrelative velocity, will always have the same sign as the relativevelocity and will always be passive. The body demand force may be activeor passive. If the body and wheel demand forces are both passive, thedemand forces are in-phase. If the body demand force is active, thedemand forces are out-of-phase.

Referring now to FIG. 19, an example flow routine for determining thetotal demand force for an actuator in the phase dependent manneraccording to this invention is set forth. The routine starts and movesto block 330 where it compares the lead-lag filtered relative velocityfor the particular corner of the vehicle to zero. If the lead-lagfiltered relative velocity is greater than zero, the routine moves toblock 332 where it compares the body demand force to zero. If the bodydemand force is less than zero, then the system is out-of-phase andblock 334 determines the total demand force (DF_(tot)) equal to the sumof the body demand force (DF_(b)) and the wheel demand force (DF_(w))for that particular wheel. If at block 332, the body demand force is notless than zero, then the body demand force and wheel demand force arein-phase and the routine moves to block 336 where it compares the bodydemand force to the wheel demand force. If the body demand force is lessthan or equal to the wheel demand force, the routine moves to block 338where the total demand force is set equal to the wheel demand force.Otherwise, the routine moves to block 340 where the total demand forceis set equal to the body demand force.

If at block 330 the lead-lag filtered relative velocity is not greaterthan or equal to zero, the routine moves to block 342 where it comparesthe body demand force to zero. If the body demand force is greater thanzero, then the body and wheel demand forces are out-of-phase and block344 determines total demand force equal to the sum of the body demandforce and the wheel demand force. If at block 342 the body demand forceis not greater than zero, then the body demand force and wheel demandforce are in-phase and block 334 compares the body demand force to thewheel demand force. If the body demand force at block 346 is less thanor equal to the wheel demand force, the routine moves to block 350 wherethe total demand force is set equal to the wheel demand force andotherwise to block 348 where the total demand force is set equal to thebody demand force. Once the total demand force is determined, theroutine exits at block 352.

The routine shown in FIG. 19 is executed for the front left, rear left,front right and rear right corners of the vehicle using the wheel demandforces, DF_(w), for the front left, rear left, front right and rearright comers of the vehicle, respectively, to determine the total demandforce for each actuator in the controllable variable force suspensionsystem. The results of block 184 (FIG. 7) are four total demand forcesignals on bus 188, one for each corner of the vehicle. Block 190performs a limiting function on the total demand force signals on bus188 responsive to the lead-lag filtered relative velocity signals, thevehicle speed signal on line 52 and the diagnostics signal on line 140.

Referring now also to FIG. 20, the demand force limiting block 190 isshown in more detail. In general, block 300 dynamically determines amaximum slope for the demand force curve and block 314 determines themaximum demand force responsive to that slope and limits the totaldemand force to that computed maximum. The limiting slope is determinedat block 302 responsive to the vehicle speed signal on line 52(V_(veh)). Separate front and rear limiting slopes are implemented.Lines 192, 194 and 196 represent the calibrations in the computer memoryof the front slope A (FSA), front slope B (FSB), rear slope A (RSA),rear slope B (RSB) and the limiting speeds SA and SB. If the vehiclespeed is below the limiting speed SA, then the front and rear slopes areFSA and RSA respectively. If the vehicle speed is greater than SB thenthe front and rear slopes are FSB and RSB respectively. If the vehiclespeed is between SA and SB, then the front and rear slopes aredetermined as linear interpolations as follows:

    Front slope=FSA+(FSB-FSA)(v.sub.veh -SA)/(SB-SA); and

    Rear slope=RSA+(RSB-RSA)(v.sub.veh -SA)/(SB-SA)).

The resultant determined front and rear slopes are output on lines 304and 306 and are provided to block 308.

Block 308 limits the front and rear slopes if a signal on line 140indicates that the ambient temperature is below a predeterminedthreshold, for example, -25 degrees Fahrenheit, in which case the frontand rear slopes are limited to predetermined low ambient front and rearslopes, respectively, represented by lines 198. (Note: At vehicle speedsgreater than, for example 100 MPH, block 308 does not limit the frontand rear slopes in response to a low ambient temperature signal on line140.) The resultant slopes on lines 310 and 312 are provided to block314 along with the lead-lag filtered relative velocity signals on lines78 and the total demand force signals on bus 188.

At block 314, a maximum demand force is computed according to the slopeon line 310 for the front and line 312 for the rear multiplied by thelead-lag filtered relative velocity signal for the two front suspensionsfor the front and the two rear suspensions for the rear. Each totaldemand force on the bus 188 is compared to the corresponding computedmaximum demand force and, if the total demand force is greater than thecorresponding computed maximum demand force, the total demand force islimited to the corresponding computed maximum demand force. This is donefor each total demand force so that the resultant output on bus 191 arenot greater than the corresponding maximum demand forces.

Those skilled in the art will understand that the above descriptionincluding block 190 represents a preferred method according to thisinvention comprising the steps of: determining an actuator demand forcecommand (on bus 188); determining, responsive to vehicle speed, a firstsignal representative of a first maximum demand force (block 302);determining responsive to ambient temperature, a second signalrepresentative of a second maximum demand force (block 308); andconstraining the determined actuator demand force command so that it isno greater than the lesser of the first and second maximum demand forces(block 314).

Referring again to FIG. 7, once the demand forces are limited at block190, the resultant demand forces on bus 191 are provided to block 200,which provides raw PWM duty cycle signals on bus 201 responsive to thesignals on bus 191, the lead-lag filtered signals on lines 78 and thehigh pass relative velocity signals on line 76.

Referring now also to FIG. 21, block 200 is shown in more detail. Atblock 316, the lead-lag filtered relative velocity signal and the totaldemand force signal for a particular corner are compared to determine ifthat corner suspension is in compression or rebound. If both thelead-lag filtered signal and demand force signal are positive, then thesystem is in rebound mode. If both the lead-lag filtered signal and thedemand force signal are negative, then the system is in compressionmode. This comparison is made at block 316 and resultant flags ofcompression, line 318, and rebound line 320, are set to indicate thequadrant of the graph in block 316 in which the corner suspension isoperating. Dotted line 322 represents that when the system is in neitherthe rebound nor the compression quadrants, but in one of the activequadrants, the damper force table is bypassed.

Block 324 represents the application of the damper force table todetermine a raw PWM duty cycle command. The damper force table isimplemented using table values 202 stored in memory, PWM duty cycles 204stored in memory and a damper force axis intercept scale factors 206stored in memory. The inputs to the table at 324 are the total demandforce and the high pass filtered relative velocity for each corner.

Referring now also to FIG. 22, the look-up table is implemented asfollows. A set of points is designated, which points are shown on thetable and correspond to the calibration points provided from memory,reference 202. Different tables are provided for rebound and compressionas designated by the flags on lines 318 and 320. For each corner of thevehicle, the raw PWM signal is determined as follows: The input highpass filtered relative velocity signal, retrieved at block 360 alongwith the total demand force signal for the corner, is compared to thecalibrated points retrieved from memory at block 362. In the exampleimplementation, there are no calibration points for the RV=0 axis, sothe RV=0 calibration points are determined at block 363 responsive tothe calibration points at RV₁ and stored slope values (axis interceptscale factors) according to:

    DF.sub.01 =DF.sub.11 -SLOPE.sub.1 *RV.sub.1,

    DF.sub.02 =DF.sub.12 -SLOPE.sub.2 *RV.sub.1,

    DF.sub.03 =DF.sub.13 -SLOPE.sub.3 *RV.sub.1,

    DF.sub.04 =DF.sub.14 -SLOPE.sub.4 *RV.sub.1, and

    DF.sub.05 =DF.sub.15 -SLOPE.sub.5 *RV.sub.1,

where DF₀₁ through DF₀₅ are the calculated calibration points at RV=0,where RV1 is the first relative velocity calibration point, where DF₁₁-DF₁₅ are the calibrated demand forces corresponding to RV1 for the fivedemand force curves and SLOPE₁ through SLOPE₅ are the five storedslopes, which may all be the same value in some implementations ordifferent values in other implementations. At block 364, the two plottedrelative velocity points immediately above and below the filteredrelative velocity signal are found and are designated as RV_(Y) andRV_(X), respectively. Block 367 interpolates between the velocity pointsRV_(X) and RV_(Y) for each of the PWM plots to find the five DF points,DF_(I) 1, DF_(I) 2, DF_(I) 3, DF_(I) 4 and DF_(I) 5 that correspond tothe particular input filtered relative velocity signal, RV, accordingto:

    DF.sub.I 1=DF.sub.X1 +(RV-RV.sub.X)(DF.sub.Y1 -DF.sub.X1)/(RV.sub.Y -RV.sub.X),

    DF.sub.I 2=DF.sub.X2 +(RV-RV.sub.X)(DF.sub.Y2 -DF.sub.X2)/(RV.sub.Y -RV.sub.X),

    DF.sub.I 3=DF.sub.X3 +(RV-RV.sub.X)(DF.sub.Y3 -DF.sub.X3)/(RV.sub.Y -RV.sub.X),

    DF.sub.I 4=DF.sub.X4 +(RV-RV.sub.X)(DF.sub.Y4 ÷DF.sub.X4)/(RV.sub.Y -RV.sub.X), and

    DF.sub.I 5=DF.sub.X5 +(RV-RV.sub.X)(DF.sub.Y5 ±DF.sub.X5)/(RV.sub.Y -RV.sub.X),

where DF_(Xa) (a=1 through 5) are the calibrated demand force pointscorresponding to RV_(X) and where DF_(Ya) (a=1 through 5) are thecalibrated demand force points corresponding to RV_(Y).

At block 368, the two of the demand forces DF_(I) 1 through DF_(I) 5that are immediately above and below the total demand force DF_(tot) areselected and designated as DF_(I) N and DF_(I) M. At block 369, the twoof the PWM curves PWM1 through PWM5 that correspond to DF_(I) N andDF_(I) M are selected and designated as PWM_(IN) and PWM_(IM). Block 370then determines the RAW PWM command as a linear interpolation betweenDF_(I) M and DF_(I) N as follows:

    RAW PWM=PWM.sub.I N+(PWM.sub.M -PWM.sub.N)(DF.sub.tot -DF.sub.I N)/(DF.sub.I M-DF.sub.I N).

The RAW PWM command is then stored at block 372 and the routine isexited at block 374. The result is a signal on bus 201 representing theraw PWM duty cycle.

Blocks 316 and 324 (FIG. 21) are performed once for each corner of thevehicle, requiring a total of up to eight tables in the preferredimplementation because different tables are used for compression andrebound. (Note: The front left and right tables may be identical and therear left and right tables may be identical, in which case there arefour tables.) In total, on bus 201, four signals are providedrepresenting the four raw duty cycle commands for the four corners ofthe vehicle.

Above is one example of how to derive duty cycle commands from demandforce. Numerous other approaches are possible. For example, a simplemapping can be implemented as follows: if DF_(tot) <DF_(A), then RAWPWM=PWMA; if DF_(tot) >DF_(B), then RAW PWM=PWMB; and otherwise RAWPWM=PWMA+(DF_(tot) -DFA)(PWMB-PWMA)/(DFB-DFA), where DFA and DFB areminimum and maximum calibrated demand forces and PWMA and PWMB are theminimum and maximum PWM commands correlating to DFA and DFB,respectively.

Referring again to FIG. 7, the raw PWM duty cycle commands on bus 201are output to block 208 where they are slew filtered in the mannerdescribed in U.S. patent application Ser. No. 08/409,411, filedconcurrently with this invention and assigned to the assignee of thisinvention. The slew filter has separate up and down limits on the rateof change of the PWM signals used to control the variable forceactuators to limit noise created by sudden changes in damper force,limit high frequency cycling of the damper valves and attenuate highfrequency components of the damper valve command.

Block 208 is responsive to the raw PWM duty cycle signals on bus 201,the gain table pointer on line 166 and the body demand force signals onbus 164. If the gain table pointer points to the isolation table, thenthe isolation up/down slew limits on lines 210 are used to limit therate of change of the PWM signal. If the gain table pointer points toany of the other tables, then the body up and down slew limits on lines212, 214 and 216 are used as the slew limits for the PWM signals on bus201. The slew factors in the downward direction may be a function ofbody demand force, for example, the greater the demand force, the longerthe slew time. The slew filtered signals are output from block 208 tooutput lines 218 containing the four slew filtered PWM signals.

Lines 218 are provided to block 220, which may be implemented if desiredto calibrate a minimum PWM command for each actuator. This minimumcommand on line 222 is used as a floor below which the duty cycle of thePWM command cannot fall. The resultant output signals on lines 84, 86,88 and 90 are the filtered PWM duty cycle commands.

According to this invention, it is recognized that the suspensioncontrol algorithm implementation shown is most accurate when the batteryvoltage input to the controller is within a small band around thevoltage levels at which the actuators were characterized for performancewhen the system is calibrated. During extremes in voltage operatingrange, the duty cycle commands to the actuators would produce currentlevels and, therefore, damper loads, that vary from the targeted levels.To limit this variation, the filtered duty cycle commands are treated asif they are accurate at a single predetermined battery voltage. Thus, athigher or lower voltage levels, the duty cycles are adjusted, or scaled,upward or downward, as appropriate, to produce the current level thatwould have occurred at the original duty cycle and the reference batteryvoltage.

The controller determines a battery scale factor responsive to theanalog battery voltage input and a four point look-up table relatingbattery scale factor calibration values to battery voltage levels. In anexample implementation, the four point look-up table has four indicescorresponding to battery levels as follows: BV1=10 volts, BV2=12 volts,BV3=14 volts and BV4=16 volts, and each indicia has a correspondingscale factor, BSF1, BSF2, BSF3 and BSF4. The scale factors are easilydetermined by a system designer by forcing the battery line to each ofthe four indices and measuring the PWM output average current for agiven set of input parameters. The measured output is then compared tothe ideal output for the set of input parameters and the ratio of theideal PWM output average current to the measured PWM output averagecurrent is the scale factor (BSF1, 2, 3 or 4) for that input voltageindicia.

Referring now again to FIG. 2, the filtered PWM duty cycle commands arescaled responsive to the battery voltage signal on line 58 at block 92as illustrated in FIG. 23. At block 462, the controller compares thevehicle battery voltage BV to the first look-up table indicia, BV1, andif BV is less than BV1, then the scale factor BSF is set to the scalefactor BSF1 at block 464. If BV is not less than BV1, then the routinemoves to block 466, where BV is compared to BV2. If at block 466 BV isless than BV2, then at block 468, the scale factor is determined as aninterpolation between BSF1 and BSF2 as follows:

    BSF=BSF2+(BSF1-BSF2)*(BV2-BV)/(BV2-BV1).

If at block 466, BV is less than BV2, the routine moves to block 470where BV is compared to BV3. If BV is less than BV3, then the routinemoves to block 472, where the scale factor is determined as aninterpolation between BSF2 and BSF3 as follows:

    BSF=BSF3+(BSF2-BSF3)*(BV3-BV)/(BV3-BV2).

If at block 470, BV is less than BV3, then the routine moves to block474, where BV is compared to BV4. If BV is less than BV4, then theroutine moves to block 476 where the scale factor is determined as aninterpolation between BSF3 and BSF4 as follows:

    BSF=BSF4+(BSF3-BSF4)*(BV4-BV)/(BV4-BV3).

If at block 474, BV is not less than BV4, then the routine moves toblock 478 where the scale factor BSF is set equal to BSF4.

The filtered PWM duty cycle commands are then scaled by multiplying eachfiltered command by the scale factor BSF to obtain the scaled PWM coedsfor the right front, left front, right rear and left rear suspensions.Additionally, if the BSF factors are not scaled 0-1, for example, if theBSF factors are scaled 0-2, then an overflow check is necessary. In theevent that the multiplication of a particular filtered PWM duty cyclecommand by the scale factor BSF causes an overflow result greater thanthe maximum duty cycle, the scaled PWM command is set to the maximumduty cycle command (i.e., 100% duty cycle).

The resultant scaled signals on lines 94, 96, 98 and 100 (FIG. 2) areprovided to block 110, the optional override block. If a signal on line66 indicates that the system is in override mode, a predetermined signalis output on each of lines 112-118 to control the actuators for testing.Also, if the diagnostics line 56 carries an error signal indicating thatthere is an error in the system, block 110 overrides the determined PWMcommands on lines 94-100 and provides a default PWM command that isscaled simply in response to vehicle speed, for example, as vehiclespeed increases, the duty cycles of the PWM commands increase. This maybe done in either a step-wise or a linear manner. The resultant controloutputs are provided on lines 112-118, which carry the duty cyclecommands for the four actuators in the suspension system and the damperlow side control command on line 120. The duty cycle commands on lines112-118 are converted in a known manner to pulse width modulated signalshaving the duty cycles commanded by the lines 112-118.

An example suitable microprocessor controller is a Motorola 68HC11 KA4,which is adapted for providing PWM output control commands. Theinterface between the microprocessor controller and the variable forcedampers may be as follows.

Assume for purposes of example that each actuator 12 controls dampingforce responsive to a continuously variable electro-hydraulic pressureregulating valve assembly of the type shown in U.S. Pat. No. 5,282,645,issued Feb. 1, 1994, assigned to the assignee of this invention. Thedisclosure of U.S. Pat. No. 5,282,645 is incorporated herein byreference. The valve responds to a pulse width modulated signal andprovides a continuously variable range of decrease in flow restrictionof a bypass passage to the reservoir of the damper between maximumrestricted flow when the valve is closed in response to a 0% duty cyclecommand and a minimum restricted flow when the valve is open andresponsive to 100% duty cycle command, or vice versa. Each example valveincludes a solenoid that, responsive to the PWM command for that damper,controls the flow restriction of that valve. A P-channel FET may be usedas the switch for the high side of the valve solenoid and the low sideof the valve solenoid is coupled via another FET controlled by outputline 120, the damper low side control. The high side P-channel FET maybe driven by an N-channel FET that is directly driven by the output ofthe microprocessor. EMI filtering in a known manner may be implementedif desired. Similar circuit control may be implemented for the low sideFET.

Those skilled in the an will understand that any suitablemicroprocessor-based controller capable of providing the appropriateactuator command for and performing the required control routine can beused in place of the example set forth herein and are equivalentsthereof.

It has been shown that this invention reduces the amount suspensionnoise generated at low speeds and at low ambient temperatures. It hasfurther been shown that this invention reduces ride harshness andimproves comfort level at low speeds. This invention obtains theseadvantages while not adversely affecting performance because a highlevel of control is still available during high speed driving maneuvers.

The above-described implementations of this invention are exampleimplementation. Moreover, various other improvements and modificationsto this invention may occur to those skilled in the art and thoseimprovements and modifications will fall within the scope of thisinvention as set forth below.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A suspension systemcontrol for use in a vehicle according to the steps of:determining anactuator demand force command; determining, responsive to vehicle speed,a first signal indicative of a first maximum demand force; determining,responsive to ambient temperature, a second signal indicative of asecond maximum demand force; and constraining the determined actuatordemand force command so that the determined actuator demand force is nogreater than the lesser of the first and second maximum demand forces,wherein vehicle body isolation from suspension events is maintainedduring low temperature and low speed operation of the vehicle to therebyminimize suspension noise and ride harshness.
 2. A suspension controlsystem for use in a vehicle including a suspended vehicle body, fourun-suspended vehicle wheels, four variable force actuators mountedbetween the vehicle body and wheels, one of the variable force actuatorsat each corner of the vehicle, and a set of sensors providing sensorsignals indicative of motion of the vehicle body, motion of the vehiclewheels, a vehicle speed and an ambient temperature, the suspensioncontrol system comprising:a microcomputer control unit including:(i)means for receiving the sensor signals; (ii) means, responsive to thesensor signals, for determining an actuator demand force for eachactuator; (iii) means, responsive to the vehicle speed, for determininga first signal indicative of a first command maximum; (iv) means,responsive to the ambient temperature, for determining a second signalindicative of a second command maximum; and (v) means for constrainingthe actuator demand force so that it is no greater than a lesser of thefirst and second command maximums, wherein vehicle body isolation fromsuspension events is maintained during low temperature and low speedoperation of the vehicle to thereby minimize suspension noise and rideharshness.
 3. A suspension system control for a suspension systemincluding a variable force damper controllable in response to a demandforce command, comprising the steps of:determining a speed of a vehicle;determining an ambient temperature outside the vehicle; determining arelative vertical velocity between a body and a wheel of the vehicle;responsive to the determined vehicle speed, determining a first gain;responsive to the determined ambient temperature, determining a secondgain; limiting the first gain to a value no greater than the secondgain; determining a maximum demand force responsive to a product of thelimited first gain and the determined relative vertical velocity; andlimiting the demand force command to a value no greater than the maximumdemand force, wherein vehicle body isolation from suspension events ismaintained during low temperature and low speed operation of the vehicleto thereby minimize suspension noise and ride harshness.